1. Field of the Invention
The present invention relates to a method for producing a prismatic secondary cell having a positive electrode core and a negative electrode core whose exposed portions protrude from end portions of the prismatic secondary cell. Particularly, the present invention relates to a method for producing a prismatic secondary cell having a current collection system structure that realizes stable current collection.
2. Description of the Related Art
In recent years, electric vehicles, such as hybrid vehicles, that use secondary cells as power sources are becoming popular. Electric vehicles need secondary cells of high output. Also for applications in mobile electronic devices, there is a need for further increase in output of secondary cells because developing functionality of mobile electronic devices.
To increase the output of cells, it is necessary to increase the opposed area of the positive and negative electrodes. The increase in the output of cells can be facilitated by using laminated electrode assembly structures in which a multiplicity of positive and negative electrode plates are laminated, or wound electrode assembly structures in which long positive and negative electrode plates are wound with separators therebetween, since the opposed area of the positive and negative electrodes is increased.
For stable extraction of current, high-output cells using laminated electrode assemblies and wound electrode assemblies employ such a structure that a current-collecting plate is welded onto exposed portions of the positive and negative electrode cores and connected to an external output terminal. From the fact that the larger the portions of connection between the current-collecting plate and the positive and negative electrode cores becomes, the larger the amount of current can be extracted stably, it is common practice to secure two or more portions of welding (see patent document 1).
However, if a plurality of welded portions are secured during resistive-welding, current expands in the horizontal direction of the current-collecting plate as shown in FIG. 24 and runs through previously welded portions. Such current becomes non-contributory current to welding, and thus it becomes difficult to realize flow of desired current through a desired welded portion. Meanwhile, increasing voltage for the purpose of securing a sufficient flow of current through a welded portion makes the welding unsatisfactory in quality such as by causing sputtering. Thus, the problem of wasted electrical energy arises.
Patent documents 2 and 3 propose such a technique that the current-collecting plate is divided into a plurality of members, which are then disposed on a single plane at an edge of a core body in a plane direction, and that each of the current-collecting plate pieces is attached with a welding electrode to realize a flow of current. However, with the technique of patent documents 2 and 3, since an edge of the core body and the current-collecting plate are welded together, the welding area is difficult to enlarge, making it difficult to sufficiently improve current-collecting efficiency. In addition, this involves a special technique of welding, which in turn poses the problem of degraded cell productivity.
The contents of patent document 1 (Japanese Patent Application Publication No. 2006-12830), patent document 2 (Japanese Patent Application Publication No. 2002-164035), and patent document 3 (Japanese Patent Application Publication No. 2002-184451) are herein incorporated by reference in their entirety.